1. Field of the Invention
The present invention relates to a power converter, and more particularly, to a power converter for calibrating its over-current mechanism according to an output of an auxiliary winding, and a related method.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional flyback AC/DC power converter 100 for receiving an AC line voltage VAC to provide an output voltage Vout to a load component 101. As shown in FIG. 1, the power converter 100 includes a bridge rectifier 105, a transformer 110, two rectifier diodes Dout and Daux, a switch element 115, a control circuit 120, a detecting circuit 125, and a compensation resistor Rcomp. Control circuit 120 controls a duty cycle of switch element 115 for controlling output voltage Vout. Control circuit 120 can detect, through detecting circuit 125, whether the current flowing through switch element 115 and transformer 110 exceeds a current limit Ilimit, that is, whether an over-current phenomenon is occurred. If the over-current phenomenon is occurred, control circuit 125 will turn off switch element 115 lest damages occur on switch element 115 or the inductors inside transformer 110.
Within a permitted voltage extent of input voltage source Vin, power converter 100 should be designed to make output voltage Vout have a fixed output power when the over-current phenomenon is occurred. When input voltage source Vin is at high line (e.g., 265V) and the over-current phenomenon is occurred, power converter 100 could operate in a discontinuous conduction mode, and thus its converted energy within a switch period can be inferred as Pt-265=½×LP×Ilimit-265V2, wherein LP represents inductance of the primary winding of transformer 110, and Ilimit-265V represents the current limit used when the input voltage source Vin is as high as 265V. However, when input voltage source Vin is at low line (e.g., 90V) and the over-current phenomenon is occurred, power converter 100 could operate in a continuous conduction mode, and thus its converted energy within a switch period can be inferred as Pt-90=½×LP×(Ilimit-90V2−IO-90V2), wherein Ilimit-90V represents the current limit used when the input voltage source Vin is as low as 90V, and IO-90V represents an initial current value of the primary winding of transformer 110. As can be seen, in order to make the equation of Pt-265=Pt-90 true, the current limit Ilimit-90V needs to be designed to be greater than the current limit Ilimit-265V. In FIG. 1, compensation resistor Rcomp is just used to make the current limit Ilimit lower if the voltage level of input voltage source Vin is higher.
Be noted that compensation resistor Rcomp effectively raises the voltage level of the positive input terminal of comparator 1250 based on the voltage rectified by bridge rectifier 105, so that the current limit Ilimit for over-current protection can be calibrated. The higher the input voltage source Vin, the higher fixed voltage level Vlift post to the positive input terminal of comparator 1250, and the smaller the current flowing through switch element 115 for triggering the over-current protection. As a result, the current limit Ilimit for detecting circuit 125 will decrease as the voltage level of input voltage source Vin increases.
However, the design for compensation resistor Rcomp damages energy-saving ability of the power converter 100, as summarized in the following:
(1) A permanent leakage path flowing from input voltage source Vin to a ground terminal is caused by compensation resistor Rcomp which results in useless power consumption permanently.
(2) Power converter 100 may not enter the power-saving mode at light load or no load with high line. The power-saving mode must be triggered only when a compensation signal VCOM (an error signal result from comparing the output voltage of output voltage source Vout with a target voltage) is smaller than a certain extent VCOM-BURST. In addition, an output voltage control mode called as a current mode is used for restricting a current peak value flowing through switch element 115 according to the output voltage of output voltage source Vout. In practice, the voltage level at the positive input terminal of comparator 1250 is compared with the compensation signal VCOM, and its comparison result controls on/off status of switch element 115. In the circuit shown in FIG. 1, regardless of light load or heavy load, compensation resistor RCOMP constantly provides the fixed voltage level Vlift to the positive input terminal of comparator 1250. In case that before being low enough to trigger the power-saving mode, the compensation signal VCOM has been smaller than the fixed voltage level Vlift, switch element 115 will be turned off constantly. In this condition, power converter 100 does not perform energy conversion, the output voltage of output voltage source Vout is not increased, and, as a result, the compensation signal VCOM cannot be decreased further to trigger the power-saving mode. That is to say, the higher the voltage source Vin, the higher the fixed voltage level Vlift, the higher the minimum voltage of the compensation signal VCOM. If the minimum voltage of the compensation signal VCOM is higher than the threshold for triggering the power-saving mode, the power-saving mode is never activated.